There are known in the prior art writing instruments formed of a body which extends longitudinally along a central axis C between a front end and a rear end. The front end comprises an opening through which a writing tip, which is not visible as it is retracted, can extend. At the rear end, the writing instrument has a push button which actuates a tip protraction/retraction mechanism. The body may be made in two parts: a conical nose and a barrel to which the conical nose is fixed.
This writing instrument is also provided with a clip used to enable the user to clip the writing instrument to a shirt pocket for example. This clip consists of an arm provided, at a first end, with a free protruding portion that must be in contact with the body of the writing instrument and, at a second end, attachment means for securing said clip to the body of the writing instrument. These attachment means may be a snap fit system, i.e. the snap fit system is inserted in an orifice in the body to secure said clip. The attachment means may also take the form of an open ring encircling the body of the writing instrument.
A first drawback is that this type of writing instrument is unattractive and fragile. Indeed, the fact that the clip is an additional part leads to risks of said clip being pulled off during handling.
Moreover, a drawback of this configuration is that it requires a manufacturing method wherein the body and the clip are each made separately. A supplementary assembly step is then provided to fit each writing instrument with a clip.
Further, the material forming the clip is any metal. Each material is characterized by its Young's modulus E or modulus of elasticity (generally expressed in GPa), characterizing its resistance to deformation. Further, each material is also characterized by its elastic limit σe (generally expressed in GPa) which represents the stress beyond which the material deforms plastically. Thus, it is possible, for a given thickness, to compare materials, by establishing the ratio of the elastic limit to the Young's modulus σe/E, for each material, said ratio being representative of the elastic deformation of each material. Thus, the higher the ratio, the higher the elastic deformation of the material. However, crystalline materials such as those used in the prior art, for example, the alloy Cu—Be, which has a Young's modulus E equal to 130 GPa and a typical elastic limit σe value of 1 GPa, give a low σe/E ratio, namely of around 0.007. These crystalline alloy parts consequently have limited elastic deformation. In the case of a writing instrument clip, it is noted that the user tends to handle the clip frequently and the clip is eventually deformed or breaks.
Similarly, the use of precious crystalline metals to manufacture such a clip cannot be envisaged given the insufficient mechanical characteristics of these metals. Indeed, these precious metals have, in particular, a low elastic limit, of around 0.5 GPa for alloys of Au, Pt, Pd and Ag, compared to around 1 GPa for the crystalline alloys conventionally used. Given the modulus of elasticity of these precious metals, which is around 120 GPa, a ratio σe/E of around 0.004 is achieved. However, a high ratio σe/E is necessary to produce such a clip, as explained above. Consequently, those skilled in the art are not inclined to use these precious metals to produce such a clip.
Another drawback of existing writing instruments is the spring which is placed in the lower portion of the body and which provides the return force to protract/retract the writing tip. Indeed, this spring is a separate part which sometimes escapes from the body of the writing instrument when the writing tip is replaced, which eliminates a basic function of the writing instrument.